Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into more desirable gases. When the engine is operated in a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.7 and 14.4, catalysts containing precious metals like palladium and rhodium are able to efficiently convert all three gases simultaneously. Hence, such catalysts are often called "three-way" catalysts.
It is desirable, however, to operate gasoline engines under "lean-burn" conditions where the A/F ratio is greater than 14.7, generally between 19 and 27, to realize a benefit in fuel economy. Such three-way catalysts are able to convert carbon monoxide and hydrocarbons but are not efficient in the reduction of NOx during lean-burn (excess oxygen) operation. Efforts have been made in developing lean-burn catalysts in recent years. One deficiency of some of the conventional lean-burn catalysts is that they are based on zeolite materials which are less than durable at the elevated temperatures necessary for their efficient catalytic operation in the exhaust gas system. Lean-burn catalysts act to reduce the NOx through the use of hydrocarbons and carbon monoxide over a catalyst, the hydrocarbons and carbon monoxide hence being in turn oxidized.
Recent efforts to solve the problem of NOx in lean-burn systems have focused on lean-NOx traps, i.e., materials which are able to absorb nitrogen oxides during lean-burn operation and are able to release them when the oxygen concentration in the exhaust gas is lowered. Hence, these traps are used with engine systems which operate primarily in a lean air/fuel ratio, but then when it is desired to purge the traps of NOx, the exhaust entering the trap is made richer, particularly rich of stoichiometric. Typical of catalyst materials used in conventional traps are an alkaline earth metal like barium combined with a precious metal catalyst like platinum. The widely held mechanism for this absorption phenomena is that during the lean-burn operation the platinum first oxidizes NO to NO.sub.2 and the NO.sub.2 subsequently forms a nitrate complex with the other material, e.g., the barium. In the regeneration mode as during a stoichiometric or rich environment, the nitrate is thermodynamically unstable, and the stored NOx is released. NOx then catalytically reacts over the platinum with reducing species in the exhaust gas like HC and CO to form O.sub.2 and N.sub.2. Hence according to one strategy for using lean-NOx traps, a hybrid-mode engine strategy is used to cycle the air/fuel ratio between extended periods of lean operations where the traps sorb NOx emissions, alternated with brief, fuel-rich intervals to desorb the adsorbed NOx and regenerate the lean-NOx trap. U.S. Pat. No. 5,473,887 discloses such operation of an exhaust purification device, the teachings of which are hereby expressly incorporated by reference herein.
The alkali metal and alkaline earth metals which are typically utilized for NOx sorption loaded on a porous support material, however, the serious drawback that they are readily poisoned by sulfur in the exhaust gas. Most fuels for automotive vehicles contain sulfur and when burnt, the sulfur is converted to sulfur compounds like SO.sub.2. Over time, the sulfur compounds react with these alkali metal or alkaline earth trap materials forming sulfates which will not revert back to the sorption material. These sulfates are inactive for NOx sorption. The alkali metals are particularly problematic. As a result, the typical NOx trap material which uses precious metal and an alkaline earth like barium is strongly deactivated by sulfur in the fuel. European Patent Application 0613714A2 published Sep. 7th, 1994 proposes a solution to sulfur poisoning of such catalysts. It discloses that the catalyst should include a porous support like alumina and loaded thereon, platinum and/or palladium, and in one aspect, also at least two alkaline earth metals like Ba, Mb, Ca, or Sr. According to the application, by having more than one alkaline earth metal, the sulfur oxides react with the alkaline earth metals forming composite sulfates that decompose at temperatures sufficiently low to avoid poisoning the catalyst. Such catalysts have the deficiency that the alumina is subject to significant loses in surface area with the thermal cycling that takes place during operation.
According to the present invention, we have now found a NOx trap material which is resistant to sulfur poisoning, has excellent NOx conversion, and also has excellent thermal stability.